A magnetic heater generates heat by a phenomenon known as magnetic inductive heating. Magnetic inductive heating occurs in an electrically conductive member when exposed to a time-varying magnetic field. The varying magnetic field induces eddy currents within the conductive member, thereby heating it. An increase in the magnitude of the variations of the magnetic field increases the rate at which the conductive member is heated. The heated conductive member can then be used as a heat source for various purposes. The heated conductive member is often used to heat a fluid, such as air or water, that is circulated past the conductive member. The heated fluid is then used to transfer the heat from the heater for external use.
One method of exposing a conductive member to a varying magnetic field is to move a magnetic field source relative to the conductive member. This motion may be achieved by arranging magnets around the edge of a circular disk having a rotatable shaft substantially at its center, the flat surface of the disk being opposable to an essentially flat portion of the surface of the conductive member. As the shaft of the disk is rotated, the magnets move relative to the surface of the conductive member. A given point on the conductive member is exposed to a cyclically varying magnetic field as each of the magnets approach, pass over, and retreat from that given point.
The amount of heat induced within the conductive member depends on many factors, some of which include the strength of the magnetic field, the distance between the magnets and the conductive member (referred herein as the “conductor/magnet spacing”), and the relative speed of the magnets to the conductive member.
Conventional magnetic heaters suffer from several disadvantages. For example, many conventional magnetic heaters have limited precision in their control of operational parameters such as the rate of heat generation, the flow rate of a working fluid used to carry heat, and the temperature of that working fluid. In particular, it is difficult to control these and other operational parameters independently from one another with conventional magnetic heaters.
For a given conductor/magnet spacing and a given magnetic field strength, increasing the disk rotation speed increases the rate of cyclical variation of the magnetic field at a given point on the conductive member, thus increasing the heating of the conductive member. Therefore, in order to be able to vary the heating of the conductive member, and thus have a range of heat output from the heater, the rotation speed of the disk must be variably controlled. For example, if a motor is used as the energy source driving the shaft to rotate the disk, the motor speed must be variable in order to vary the heating of the conductive member. If a windmill is used as an energy source, it may be very difficult to selectively change the rotation speed of the disk.
In order to produce a constant heat output from the heater, a constant rotation speed of the disk must be maintained. If an internal combustion engine is used as the energy source driving the shaft to rotate the disk, the engine must be throttleable to produce a constant output at a given tachometer setting, in order to induce a constant heating of the conductive member. A power takeoff from a vehicle engine, such as a tractor engine, commonly has a constant speed of rotation for a given throttle setting. Although it is possible to change the heat output by varying the engine speed, doing so may effect the vehicle in undesirable ways. If a windmill is used as an energy source to rotate the disk, it is very difficult to produce a constant rotation speed of the disk.
Some magnetic heaters utilize a rotating disk designed with a preferential shape to provide the driving force for circulating heat-transfer fluid. Alternatively, the shaft is used to drive a fluid pump. Consequently, the fluid flow rate is directly determined by the rotation speed of the shaft. Driving the fluid in this manner makes it difficult to control the temperature of the fluid exiting the magnetic heater. As the shaft speed is increased in order to increase the heating of the conductive member, the flow rate of the fluid is consequently also increased, which, in-turn, works against obtaining a desired fluid temperature for a given flow rate.
Although it is possible to address the difficulty of controlling fluid flow and/or fluid temperature by driving the fluid with a mechanism separate from that used to rotate the disk, reliance on such an additional mechanism tends to increase the size, weight, and complexity of the magnetic heater.
The disadvantages of limited control of operational parameters impact the usefulness of the magnetic heater. For example, many types of grain are routinely dried after harvesting by exposure to a flow of heated air. The appropriate temperature to which the air should be heated and the preferred rate of air flow depend on many variables, such as the amount of moisture in the grain, the type of grain being dried, and the quantity of grain present. A conventional magnetic heater that cannot reliably maintain operating parameters at their desired levels, or that cannot readily be adjusted to reach those desired levels, is of limited use in drying the grain.
Therefore, a magnetic heater is needed that facilitates control over various parameters, in combination or individually.